This invention relates to a thermal energy storage composition with good stability and physical properties, especially useful for space heating applications where it is incorporated in either active or passive thermal energy storage systems. The objects of such storage systems are to provide operating efficiencies to thermal operations, to achieve energy savings, and to permit the introduction of new, alternative energy sources, such as solar energy, that are characterized by intermittent availability. Individual tailoring of each storage system is required to match the requirements of each of the potential applications. Current, commercial systems employ the specific heat capacity of water, rock, or ceramics to store the heat. However, large volumes, large temperature swings, and bulky, expensive heat exchange apparatus are required.
Commercialization has recently begun of systems employing phase-change materials to store the heat. These materials store significant quantities of energy upon melting. Conversely, this energy is liberated when the liquid is solidified. A number of these phase-change materials are disclosed in U.S. Pat. No. 3,986,969 such as calcium chloride hexahydrate, sodium carbonate decahydrate, disodium phosphate dodecahydrate, calcium nitrate tetrahydrate, sodium sulfate decahydrate and sodium thiosulfate pentahydrate.
A candidate phase-change material that has received research and development effort over several decades is Glauber's salt, sodium sulfate decahydrate. Its heat of fusion is high, its melting point is appropriate for many space heating applications, and its price is low; however, Glauber's salt suffers from loss of its water of hydration, separation into an anhydrous sodium sulfate layer and a water layer, and a consequent loss of ability to store heat. Another serious problem is its tendency to supercool, releasing thermal energy in large quantities at undetermined times. Many modifications by way of additives and container configurations have been tried in order to overcome these problems. Thus, the patentee in U.S. Pat. No. 2,677,664 disclosed and claimed the use of borax as a nucleating agent to prevent supercooling in a thermal storage composition based on Glauber's salt. The same patentee added a homogenizing or thickening agent such as attapulgus clay to the same composition in U.S. Pat. No. 3,986,969 in order to slow the rate of separation.
Use of thin tray containers allows convective currents to help keep the components mixed. A rotating drum has been tried to prevent component separations. Both indirect and direct-contact thermal exchange arrangements have been demonstrated. Despite these efforts, sufficient problems remain with phase-change candidates so that significant commercial use of any of them has not occurred.